In Nicole Anderson's book, The Glory of Beads, which I just flipped open looking for something else, I found an image of a bead sample card that clearly shows four different oval beads with spiral, twisted cane decoration that is not melted into the surface. None are exactly the same as my beads, but I feel much more confident that they are indeed old Venetian production.

The exciting thing about old beads is that even after almost ten years of looking at thousands (or more!) of different Venetian beads, I still find ones that I haven't seen before!